The present invention is a method for sintering and/or melting particles such as nano-, micro- or quantum dot particles to form a continuous film using electron beams (Ebeams). These Ebeams may be produced through ordinary, non-accelerated electron beams, normal conducting linear accelerator (LINAC) Ebeam systems or through superconducting linear accelerator (scLINAC) Ebeam systems. scLINACs are discussed below as an exemplary case with added benefits above other systems (i.e. rate and process depth). Particularly, the present invention is concerned with a process for fabricating bodies such as thin films composed of semiconducting powders or inks used as photovoltaic or other optoelectrical devices. These materials may include, but are not limited to, silicon, germanium, cadmium selenide, cadmium telluride, copper indium gallium selenide (CIGS), copper indium selenide (CIS), copper sulfide, copper zinc tin sulfide (CZTS), lead sulfide, and lead selenide among others familiar to those of ordinary skill in the art. The materials may be in micro- or nanoparticle form.
The present invention is capable of overcoming many of the hurdles associated with thermal annealing, photonic curing, or laser sintering of semiconductor films. Unlike lasers, Ebeams are transparent to plasma and are capable of homogeneous heating, X-ray dosing, and electric field generation at the surface and the subsurface of a material to a predetermined depth. When laser processing power is increased, plasma is generated, reflecting the laser and resulting in spotty, unprocessed regions. The higher energy and power of Ebeams far exceeds laser and photonic curing, allowing faster processing and heating speeds. Faster processing speeds and higher energy reduces costs by allowing for more rapid production. This is especially important for photovoltaic technology since cost per kW is the largest prohibitive feature for widespread implementation of this technology.
In order for photovoltaic (PV) devices to generate energy competitively on a cost basis with carbon-based fuels, bodies such as PV films must become more efficient and/or cheaper. Much effort has been dedicated to achieving higher power conversion efficiencies, often at the expense of more complicated and expensive materials. Another route is to pursue low-cost manufacturing methods for PV production. In particular, this means foregoing traditional epitaxial and other non-scalable and costly fabrication methods for low-cost solution processing. For example, powders or inks composed of micro- and nanocrystalline semiconductors can be tape cast, direct-write, printed, painted, or spray-cast on substrates in established low-cost, high-volume industrials processes. The advantage of these powders or inks is that they can then be deposited on flexible substrates for use in conformal photovoltaic or optoelectrical devices which may be integrated into many applications such as wearable electronics.
The as-deposited films are, however, often very poor conductors of charge as the individual semiconductor particles are electronically isolated from one another by surface ligands required for solution suspension. The particles suspended in the ink require surfactants to prevent aggregation and precipitation from the solution prior to deposition. The surfactant molecule can be electrically conductive or semiconductive, causing shorting across the film, but is usually electrically insolating, increasing film series resistance. To produce a cohesive and electrically-connected film without shorting, the as-deposited particles are exposed to energy, typically in the form of heat or light, to destroy the ligand and sinter the particles together into a polycrystalline film. These approaches include photonic curing, rapid thermal annealing, and laser sintering. In all cases, the energy is converted to heat in the crystal which destroys or volatilizes the surfactant groups and sinters the particles together into a cohesive film.
This electrically-converted heat is not localized to the inter-particle interface, however, and is typically conducted to the entire device structure. Even in the case of localized laser sintering, the heat is conducted to the surrounding layers and to the underlying substrate. This can prevent flexible substrates, like polymers, from being used as they will melt and deform at temperatures much lower than the necessary sintering temperature of semiconductors. Furthermore, laser sintering creates plasma at the film surface which reflects the light and inhibits film heating. Thus the laser must scan at a slower speed and lower energy to ensure that enough heat is generated in the film, thereby increasing the processing time and leading to higher thermal spreading. This higher time characteristic associated with the laser scan speed means longer processing times, increased heat flux to the substrate, and even can cause ion migration in the crystal structure. In some cases, the energy required to destroy the ligand is prohibitively high such that photocuring is not possible due to extreme plasma generation and light reflection.
Stolle et al. [J. Phys. Chem. Lett., 2014, 5, 3169-3174] demonstrated that exposure to excessive photonic curing leads to decreased photovoltaic performance. The photonic curing process is very non-uniform at high power. The cause was not explained in that paper, but it is likely the result of plasma having been generated during the sintering process which caused the photons to be reflected from the surface. We recognized that Ebeam sintering would not have this problem as electrons are transparent to plasma and would not be reflected. In addition, laser sintering only heats the sample surface and relies on heat conduction for sub-surface heating. Ebeam processing would more rapidly heat the surface and sub-surface since the beam can penetrate through the material and impart energy. Rapid localized volumetric heating with Ebeams has the added benefit of lower cost through faster and out-of-vacuum processing. In addition, rapid local homogenous heat to the film can allow for processing on flexible polymer substrates to create wearable, low-cost electronic devices and solar cells.
The present invention is directed to a process that sinters semiconducting films through highly localized, rapid heating via Ebeam sintering and/or melting and to the products of that process. Specifically, ultra-high energy and power scLINAC-produced beams have been found to stand apart from existing methods in that these systems are capable of higher electron energy and power. Beam energy directly scales with process depth, which can be dramatically deeper with scLINACs. Beam power, which is at least 20× higher with scLINACs than any other system, scales with process speed (heating rate). High-energy Ebeams using a scLINAC are utilized to provide continuous electron beam exposure to the sample piece. The concept for Ebeam-LINACs was first introduced as the Betatron in the 1940's. See, e.g., U.S. Pat. No. 2,394,072. Only recently, however, have high-power Ebeam-scLINACs been recognized as a viable option for unique materials processing due to reductions in system size, power consumption and cost. The ability of the Ebeam-scLINAC to precisely deliver energy to a prescribed volume of material is unique, and we have discovered there are a substantial number of materials-processing operations now possible with its use as described in U.S. Pat. No. 9,328,976. As a way to succinctly illustrate the principles of the present invention and its advantages, the following discussion compares thermal material processing with Ebeam-scLINACs; specifically with respect to the production of CIGS films, and, more specifically, the production of solution-processible, flexible photovoltaic devices. Processing of thin films can be accomplished, however, with conventional conducting and scLINAC Ebeam systems alike. scLINAC-produced Ebeams will be discussed going forward only as an exemplary case.
Energy delivery to material via a scLINAC-produced Ebeam is both rapid and efficient with nearly 100% of the electron energy being imparted to the material. In contrast, conventional thermal processing by contact, convection or irradiation heating is slow, and a large amount of energy is lost to the surroundings or to heat up the instrument itself. These known methods do not allow for simultaneous heating of both the workpiece material's surface and subsurface without undesirably heating support or balance of device components, especially with any degree of control with respect to subsurface processed depth. Furthermore, laser irradiation often generates plasma that reflects light. Plasma is also generated with such Ebeam irradiation, but the electrons are transparent to plasma so that the thermal processing can continue uninhibited.
Electron beam heating produced with the scLINAC is optimal for rapid, selective, localized heating. The internal energy generation is localized to a region, and the size of the region depends on the beam parameters, including accelerating voltage and beam diameter, and the properties of the target material. Unlike resistive heating, the energy generation is not uniform and is limited to a region adjacent to the surface regardless of electrical conductivity. Electron beams so generated can deliver a large amount of thermal energy in a small region with efficiencies of greater than 90%, making it an optimal solution for the fast sintering of semiconducting films. Furthermore, most Ebeam systems operate in a pulsed capacity with on and off cycles to prevent the machine from overheating. However, the Ebeam-scLINAC operates continuously with a 100% duty cycle. As a result, has a power density at least twenty times higher.
The present invention has the advantage of being able to fabricate continuous polycrystalline films with improved electrical and thermal properties from powders or inks. Specifically, the powder beds or printed inks are sintered into a cohesive film through ultra-high energy/power electron beam sintering. Of specific interest are films of semiconducting materials. These materials may include, but are not limited to, silicon, germanium, cadmium selenide, cadmium telluride, CIS, CIGS, copper sulfide, CZTS, lead sulfide, and lead selenide among others familiar to those skilled in the art. These semiconducting films can replace expensive single-crystalline layers in photovoltaic devices, thermophotovoltaics, infrared detectors, focal plane arrays, and emitters in light-emitting diodes or lasers among others. Of course, it should be understood that the present invention is not limited to such films but may include other types of bodies and materials.
An object of our invention is to lower the manufacturing cost of products like single-crystalline semiconductor layers for photovoltaics and other optoelectrical devices. The solution processing of a semiconductor ink, in particular, can be integrated into existing printing lines well-known to those with knowledge in the art. Similar processing can be achieved with powders or slurries of semiconductor particles to form the films. Our high energy Ebeam processing approach may also be integrated into the roll-to-roll assembly line. The Ebeam-scLINAC would continuously raster the films sintering the printed or cast particles into a cohesive film and pass the films to the next step in the assembly line. This continuous assembly-line processing is a stark contrast to the epitaxial growth of single crystal films employed in many photovoltaic devices.
A further object of this invention is to improve the electrical conductivity, thermal conductivity, and charge transport of the semiconductor film. The untreated powder or ink contains surfactants and additives that prevent aggregation of particles and allow for uniform deposition. As above noted, these largely organic surfactants and additives are electrically insulating and inhibit charge transfer. Conductive or semiconductive surfactants would electrically short a device and usually need to be removed as well. Our high energy Ebeam processing approach removes the surfactant groups from the surface of the particles and sinters or melts the particles together to form physical contact. This type of processing dramatically increases the electrical and thermal properties of the cohesive film and improves the performance of the photovoltaic and optoelectrical device.
Another object of our invention is to reduce thermal spreading of the Ebeam processing by scanning the electron beam across the surface of the body. A small and controllably localized region at and below the surface is heat treated as the beam moves. The rate at which the energy is deposited exceeds the rate of conduction away from the region, allowing the semiconducting or other material at sufficient depth from the surface to remain below a critical temperature. The local heating allows the semiconducting film to be sintered directly on a flexible polymer substrate without actually melting the polymer. Active cooling of the substrate can also be used to maintain substrate temperatures below the polymer melting or softening temperature. The selective heating of the film is a unique feature of Ebeam-scLINAC processing of semiconductor powders or inks that provides alternatives in the choice of substrates to rigid metals or silicon wafers. For example, the ability to process there semiconductor films is advantageous for flexible optoelectronic devices for use in conformal applications, integration with textiles, and wearable electronics.
Still another object of this invention is to improve the processing speed of the film sintering process. Laser sintering or photonic curing can lead to plasma generation at the surface of the film which scatters much of the incoming energy before it can reach the sample. This means that either the laser power or the processing time or both must be increased to assure enough energy is transferred to the film for effective sintering. In some cases, photonic processing is not even possible due to the balance between the target temperature and plasma generation. This can lead to unsintered powders and/or incomplete breakdown of the ligand. Either result increases the series resistance of the device. While our Ebeam processing approach also generates plasma as the film surface, the electron beam is transparent to the plasma. Therefore nearly all of the Ebeam energy is transferred to the film. Furthermore, as above noted, Ebeam-scLINAC systems operate continuously with a 100% duty cycle and therefore are capable of faster rastering speeds as more energy is imparted to the film in a shorter amount of time compared to normal Ebeam processing.
The present invention thus dramatically improves on the known variations of producing semiconductor films and interlayers by allowing for low cost, solution processing of photovoltaic and other optoelectronic devices. The processing time is reduced by high energy Ebeam-scLINAC processing from faster rastering speeds, and the selective heat treatment is localized compared to lower energy Ebeam processing or laser sintering. The reduction in thermal spreading allows for the use flexible polymer substrates for conformal applications such as wearable electronics.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings and non-limiting examples herein.